Fuel Demand Valves: Troubleshooting the Valve That Holds Back a Sealed Tank

A traditional anti-siphon valve uses a spring-loaded ball. It stays closed against the weight of fuel trying to siphon out of a broken line, and the fuel pump's suction pulls it open. That works on a tank vented to the air.

On a sealed tank, the pressure pushes in the same direction as the fuel flow, so it can help push a ball valve open. A demand valve's diaphragm is pulled open only by the engine's suction, not by tank pressure. Perko's version, for example, is made to stay sealed against tank pressure above 5 psi, open at a slight vacuum (under 1 psi), and still provide anti-siphon protection for a limited drop in the fuel line below the tank top (12 inches on its standard model).

That's the key point of this guide: the two valves are not interchangeable.

A demand valve fails in one of two ways:

• Stuck closed or opening too hard (stiff or swollen diaphragm, debris, wrong valve): the engine starves for fuel. It may idle fine but stumble or die at higher speed or under load, then restart after sitting a few minutes. On outboards, the primer bulb may collapse while the engine runs.
• Leaking through (torn diaphragm, debris holding it open, or no valve at all): pressure pushes fuel to the engine while it's off. Look for hard hot starts, a strong fuel smell, fuel dripping from the cowl or carburetor, or a rising oil level on a four-stroke (gasoline in the oil).

Owners often discover the second one after buying a new EPA tank and using their old fuel line with no demand valve. A new tank that swells in the sun and an engine that floods suggest exactly that.

Neither symptom proves the valve is bad. Work through the cheaper checks first.

• Portable tank: most EPA tanks have a manual vent on the cap. Open it before connecting and while running if the tank maker says to. A tank that's sucked in (dented inward) while running points to a vent problem; one that's bulging points to pressure the valve should be holding back.
• Built-in tank: remove the fill cap and listen. Practical Sailor notes that a puff of air on a freely vented pre-2012 boat signals a vent restriction; on newer sealed or canister systems, some pressure can be normal. Check the vent fitting on the hull for mud-dauber nests, paint or a crushed hose.

Squeeze the bulb with the engine off. It should firm up and stay firm. A bulb that never firms up, or a quick-connect fitting that leaks or sucks air, can mimic a valve problem. Check that the bulb's arrow points toward the engine; BoatUS warns that a reversed bulb keeps the engine from starting. Our primer bulb guide covers replacement.

A clogged filter starves the engine exactly like a stuck valve. Replace the fuel-water separator element (and inspect the bowl for water) before suspecting anything more expensive. See our fuel-water separator guide.

BoatUS ran into a sneaky one while installing a demand valve: the inner plastic liner of a newer ethanol-resistant fuel hose had separated, and pushing the hose onto the valve's barbs shoved the liner back and blocked the flow. Look for hoses that are hard, cracked, swollen, kinked or soft and collapsing under suction. If in doubt, replace the hose, or use a pre-rigged fuel line with the valve already installed.

If you have a portable tank and line that you know work, with a working demand valve, run the engine from it in the water or on external flush muffs (never dry). If the problem disappears, the fault is between the original tank and the engine: vent, valve, line or bulb. If it stays, look at the engine's own fuel system instead.

A technician can put a vacuum gauge in the line between the valve and the engine's fuel pump and run the engine under load. High vacuum with a clean filter and good hoses points to a restriction at the tank, which on a built-in tank is often the demand valve or a clogged pickup screen below it. Compare readings with the engine maker's service limits. This is the clearest way to prove the valve is at fault before replacing a tank-mounted one.

• In-line valve: look for a small housing in the line near the tank with an arrow showing flow direction. The arrow must point toward the engine.
• Tank-mounted valve: on top of a built-in tank where the fuel line leaves it, often under a deck hatch. Perko and other makers build them flanged (bolted like a fuel gauge sender) or threaded into the tank.

Photograph the brand and part number. That tells you exactly what to replace it with.

BoatUS describes this as a simple job:

1. Disconnect the line from the tank and engine, and drain it into a glass jar (disconnecting the primer bulb helps, since its check valve stops backward flow).
2. Cut the line between the tank and the primer bulb, at least 8 inches from the tank, if you're adding a valve, or remove the old valve.
3. Fit the valve with the arrow toward the engine, push the hose fully onto each barb and secure it with a stainless clamp.
4. Reinstall the primer bulb the right way around.

Many owners find a factory pre-rigged fuel line with the valve built in is easier and avoids hose liner trouble. If the old gas looks clean, BoatUS says it can go back in the tank.

On some portable tanks the valve isn't in the line at all: it's part of the threaded outlet fitting that screws into the tank. Mercury's Quicksilver catalog, for example, sells a tank connection with a built-in demand valve that is listed for specific Quicksilver/Mercury tank sizes and described as meeting the EPA requirements for pressurized portable tanks. If yours is that type, buy the fitting listed for your tank's brand and size rather than adding a second valve in the line.

A valve on a built-in tank is part of the boat's certified fuel system. Replace it only with the same type and part (or the replacement the tank maker or boat builder specifies), fitted with the correct thread sealant or gasket. Hold the tank fitting so it can't turn while you loosen the valve, and work with the tank well below full. If the valve is hard to reach, corroded, or the tank fitting looks damaged, this is a job for a marine technician.

It's tempting to swap a balky demand valve for a cheap ball-type anti-siphon valve, or to pull the valve out altogether. Don't:

• A ball valve can't reliably hold back sealed-tank pressure, so you're back to fuel pushing into the engine and, on an inboard or sterndrive, possibly into the bilge.
• The Coast Guard's rule (33 CFR 183.568) still requires anti-siphon protection, an electric fuel stop valve, or approved manual shut-offs on built-in tanks where the line runs below the tank top. On many boats, the demand valve is that protection.

Reconnect, squeeze the primer bulb (outboards) or turn the key to let an electric pump prime (follow your engine manual), and inspect every new connection with a dry white rag before starting. On an inboard or sterndrive, run the blower for at least four minutes and sniff the bilge before starting, as BoatUS recommends. Any drip or smell: shut off, fix it, and check again.

Run at idle, then cruise, then wide open for a few minutes. The engine should hold speed without surging or dying, and an outboard's primer bulb should stay full. Afterward, let the boat sit in the sun with the engine off and check for fuel smell or drips at the engine. That confirms the valve is holding back the tank pressure.